Process for the production of metallic niobium or tantalum by the electrolysis of melts



United States Patent PROCESS FOR THE PRODUCTION OF METALLIC NIOBIUM OR TANTALUM BY THE ELECTROLY- SIS OF MELTS Kurt Huber, Berne, Remy 'Chaperon, Sierre, and Fritz Kern, Binningen, Switzerland, assignors to Ciba Limited, Basel, Switzerland, a firm of Switzerland No Drawing. Filed Aug. 12, 1957, Ser. No. 677,796

Claims priority, application Switzerland Aug. 14, 1956 10 Claims. (Cl. 204-64) This invention relates to the production of metallic niobium or tantalum by the electrolytic method, and provides such a process in which so-called oxygen-free, that is to say oxide-free, fluoride-containing melts are used.

It is known to use as an electrolyte for the electrolytic production of metallic tantalum an alkali-metal halide melt which contains tantalum pentoxide (Ta O and potassium fluoro tantalate (KgTaFq) dissolved therein. The anode usually consists of graphite and the cathode of graphite, iron or nickel. The empirical reaction which takes place in the cell may be represented as follows:

Ta O CO This method of production has two disadvantages each of which has an unfavorable effect on the purity of the product. Thus, on the one hand it is very difficult to add the quantities of tantalum pentoxide to be incorporated in the electrolyte to replace the electrolytically deposited metallic tantalum in such a way as to avoid a part of the oxide becoming fixed in the cathode deposit. Oxide, which becomes fixed in the deposited tantalum in this manner, can be removed from the metal only with difficulty or not at all. The properties of the metallic tantalum separated from the cathode depend to a very great extent on the purity of the metal, and only if the metal is substantially free from oxygen does it possess the desired ductility and workability. The second and more serious disadvantage is that the particles of graphite resulting from the unavoidable burning away of the anode enter the electrolyte and, as the electrolyte is subjected to considerable movement by the evolution of the anodic gas, reaches the cathode and enters the deposit on the cathode. The resulting carbon content of the metal powder is very harmful, because it leads to graphite formation and consequent reduction in the ductility of the metal. The anode gas, which contains carbon monoxide and carbon dioxide, is also harmful because it can react with the tantalum deposited at the cathode with formation of oxide.

In order to avoid the diificulties attendant upon the use of oxygen-containing melts it would appear desirable to use an oxygen-free, that is to say, oxide-free, electrolyte. In this case fluorine or, if the bath contains chloride, chlorine would be evolved at the anode instead of carbon monoxide and carbon dioxide. The main objection put forward in the literature against such a method is that, in the absence of oxide, the anode effect known, for example, in the electrolytic production of aluminium would inevitably occur. This anode eflect causes a considerable rise in voltage between the anode and the cathode and makes it undesirable or even impossible to continue the electrolysis. A further objection is that the current yield is very low when an oxygen-free electrolyte is used.

The present invention is based on the observation that 'tantalum powder or niobium powder of good quality and ductility can be obtained with a good to very good yield of metal, and with the avoidance of the harmful anode "ice effect and at surprisingly good current yields, by the electrolysis of an oxide-free bath, if an alkali metal double fluoride of tantalum or of niobium, and advantageously sodium and/or potassium fluorotantalate, is electrolysed in a substantially oxygen-free molten bath of diluent composed of alkali metal chloride or alkaline earth metal chloride at a temperature within the range of 650-950 C., and advantageously 750 C.900 C., at an anodic current density of at most 500 amperes per square decimetre and preferably at most 300 amperes per square decimetre, and the anodic current density is kept lower the higher the fluoride concentration of the electrolytic bath. Thus, it has been found that the fluoride content of the electrolyte is of even greater importance than the oxide content, which is repeatedly referred to in the literature, since at a given temperature the critical anodic current density, that is to say, the current density at which the anode effect occurs, should be lower the higher the fluorine content of the electrolyte. If the conditions are so chosen that the electrolysis is carried out continuously below the critical current density, metallic tantalum of very good ductility is deposited at the cathode with a good current yield and product yield. The critical current density, below which the process of the invention is carried out, can be expressed as a function of the tem perature of the melt and the fluorine concentration of the melt, only the fluorine present in the alkali metal fluoride in the melt and not the fluorine bound as fluorotantalate being taken into account for this purpose. The value of the critical anodic current density :r for the preferred temperature range of 750 C. to 900 C. and at a fluorine concentration of 4.5 to about 18% can be approximately determined from the empirical formula in amperes per square decimetre (amp./dm. where R is the temperature of the melt in degrees centigrade, e is the Euler number (the base of natural logarithms), and A is the concentration of the fluorine bound as alkali metal fluoride expressed as a percentage of the melt. This formula was derived from measurements taken with respect to the double fluoride of tantalum but it approximately also applies to the double fluoride of niobium. Thus it is found from this Equation I that with a temperature T of 750 C. and a fluorine concentration A of 4.5% (only the fluorine present in the melt as alkali metal fluoride being taken into account and not the fluorine bound as fluorotantalate) -the critical current density (o' is about 108 amperes per square decimeter. At the same temperature and a sodium fluoride concentration, for example, of 40% (corresponding to a fluorine concentration A of about 18%) the critical current density calculated from the above equation is about 14.5 amperes per square decimetre, while the corresponding values determined experimentally are 106 and 14 amperes per square decimetre, respectively. However, for the highest temperature and fluorine concentration Equation I gives a rather too low a -value.

Somewhat more accurate values for the maximum or critical current density (a are obtained from the following empirical equations which apply only to certain ranges of temperature and concentration wherein A and T have the meanings given for Equation I, and Equation II applies only for fluorine concentration within the range of 4.5% and 7% and Equation III only for concentrations above 6.8%. For a fluorine concentration of, for example, 6.9% (corresponding to a sodium fluoride concentration of about 16%) the maximum permissible current density can therefore be calculated from both Equation 11 and Equation III. Equations I, II and III provide no information as to the reaction mechanism of the electrolytic production of metallic niobium and tantalum, but they provide practical and simple means for determining the anodic current density below which it is of advantage to carry out the process of this invention. The following table gives the experimental values for the critical anodic current density in a melt of equimolecular proportions of sodium chloride and potassium chloride containing 10% of KzTaFq at 750, 800, 850 and 900 C. under the conditions prevailing in the cells used, that is to say, with the use of a graphite crucible as cathode having an internal diameter of 50 millimeters and a depth of 220 millimetres and maintained under nitrogen to exclude the atmosphere, and with the use of a graphite rod of 11 millimetres diameter as anode.

NaF, percent 10 15 30 40 =F,percent 4.5 6.8 9.0

critical current density in /dm 2 154 80 49 28 21 800 amp.

For preparing the fused electrolyte there are used as diluents for the niobiumor tantalum-alkali metal double fluoride, alkaline earth halides or preferably alkali metal halides, especially an alkali metal chloride or a mixture thereof. Thus, for example, sodium chloride, which is inexpensive and easy to obtain, may be used as the sole constituent of the diluent or, if a bath of lower melting point is desired, there may be used an equimolecular mixture of sodium chloride and potassium chloride.

As alkali metal double fluorides to be used for Ohtaining metallic niobium or tantalum by electrolysis in the molten salt bath there are used more especially sodium or potassium double fluorides such as sodium fluorotantalate or potassium fluorotantalate. These double fluorides can be obtained by simple crystallisation from aqueous solutions thereof. Usually the double fluorides so obtained are sufficiently pure and yield metallic tantalum of high purity.

The proportion of the alkali metal fluorotantalate to be used in the process may vary within wide limits. The proportion is non-critical within the range of 2 to 40% by weight of sodium fluorotantalate or potassium fluorotantalate calculated on the total weight of the melt. It is advantageous to use melts containing from 10 to 30% of'the fluorotantalate.

It is necessary to use the double fluoride and also the diluent salt in as anhydrous a condition as possible, so that the process is carried out in accordance with the invention, that is to say, as an electrolysis of an oxygenfree bath.

The process of the invention does not necessitate any modification of the customary cells used for fused electrolytes or any other modification of the apparatus used for the electrolytic production of metals from alkali metal double fluorides.

The anodic current density is advantageously chosen in the manner described above. In this connection it is to be noted that as the electrolysis proceeds the alkali metal fluoride content of the electrolyte increases in accordance with the empirical reaction formula 10KCl+2K TaF- 2Ta+5C1 +14KF In order to prevent the occurrence of the anode effect 4 due to this phenomenon either the anodic current density may be suitably chosen or the electrolyte may be diluted with alkali metal chloride.

When the electrolysis is carried out in a discontinuous manner, the termination of the deposition of tantalum is indicated by an increase of the cell voltage to the decomposition voltage of the alkali metal chloride serving as diluent. Within a few minutes after this has occurred further chlorine is evolved at the anode, but at the cathode the corresponding alkali metal, for example, sodium, is deposited.

It is of great importance in carrying out the process in accordance with the invention that the atmosphere should be excluded from the electrolytic cell. This is advantageously achieved by using a closed electrolytic vessel, in which the air above the melt to be electrolysed is replaced by an inert gas, for example, nitrogen or a noble gas, such as krypton, neon, argon or helium. In this manner the formation of oxygen-containing com pounds in the electrolyte by hydrolysis or oxidation is prevented, which compounds would decompose electrolytically to form oxygen which would attack the graphite anode with the formation of carbon dioxide and carbon monoxide.

In order to recover the deposited tantalum the oathode may be lifted out of the electrolyte bath and allowed to cool in an atmosphere of argon or neon above the electrolyte. In this way the greater part of the electrolyte adhering to the tantalum tree flows back into the cell. After this cooling operation, the tantalum powder is freed from the residues of adherent electrolyte by washing with waterand dilute mineral acid in known manner, and dried.

The powder obtained by the process described above has a fine to coarse granular structure depending on the conditions of electrolysis, the composition of the bath, the current density and the electrolysis temperature. The powder is characterised by an excellent ductility and can easily be pressed into bars, and converted into compact metal by known methods.

By the process of this invention it is therefore possible to obtain metallic tantalum of good ductility from oxygen-free melts at good current yields and product yields, and to avoid occurrence of the anode effect by suitably selecting the conditions of electrolysis. By maintaining the anodic current density sufficiently low depending on the temperature and the fluoride concentration of the bath, the electrolytic decomposition of the alkali metal chloride used as diluent commences, after the deposition of tantalum, without disturbance by an anode effect.

The double fluoride of niobium can be subjected to electrolysis in a similar manner to that of tantalum. The tendency of niobium to form stable lower valency states is troublesome because the products of lower oxidation stages formed at the cathode are soluble in the melt and, for example, due to convection reach the anode where they are reoxidised.

This phenomenon which, of course, considerably reduces the current yield can be counteracted, for example, by separating the anode and cathode zones from one another either by a diaphragm or by the provision of simple obstructions which restrict convection.

The following examples illustrate the invention:

Example 1 A melt consisting of grams of K TaF dissolved in 210 grams of an equimolecular mixture of sodium chloride and potassium chloride was subjected to electrolysis in an electrolytic cell consisting of a graphite crucible of 50 millimetres internal diameter and 220 millimetres in height serving as anode and a nickel rod of 11 millimetres diameter serving as cathode, which rod is protected at the portion thereof not immersed within the electrolyte melt against the action of chlorine by a graphite tube, and having a quartz crucible enclosing the whole against the access of the atmosphere. The bath temperature was 750 C. and the current strength 25 amperes. Under the conditions used the voltage across the anode and cathode leads was initially 3.40 volts. In addition to the actual voltage of the bath, the aforesaid voltage also included the resistances at the junctions, for example, from the lead to the graphite crucible. The anodic current density at a bath depth of 90 millimetres was about 15 to -17 amp./dm. After a period of electrolysis of 80 minutes the voltage had increased to 3.78 and after 95 minutes to 3.84 volts. The voltage then increased to 4.12 volts in the course of 2 minutes, thus indicating that the deposition of tantalum had ceased and the transition to the electrolysis of pure alkali metal chloride had commenced. At this point the cathode was lifted out of the electrolyte into the upper part of the electrolytic cell and allowed to cool in a current of argon.

The metal powder obtained after removing it from the nickel cathode and washing it with Water and dilute acid weighed 36.1 grams. This weight corresponds to a product yield of 87% and a current yield of 68%.

The metal powder was of medium fineness and very ductile.

Example 2 A melt consisting of 30 grams of K TaF and 270 grams of NaC1-KC1 eutectic was electrolysed in the cell described in Example 1 at 800 C. and a current of 25 amperes, corresponding to an anodic current density of about 16 amp./dm. After 28 minutes an increase in voltage of about 0.3 volt occurred within a short time, and then the electrolysis was discontinued in the manner described above.

The isolated tantalum powder had a fine to very fine structure and was ductile. The yield of product was 72.5% and the current yield 67% Example 3 A melt consisting of 60 grams of 'KgTaFq and 240 grams of NaCl-KCl eutectic was electrolysed in the cell described in Example 1 at 850 C. with a current of 25 amperes. After 59 minutes the increase in voltage, which accompanies the transition from the deposition of tantalum to the deposition of alkali metal, took place and then the electrolysis was discontinued.

The isolated tantalum powder had a medium coarse to coarse grain size and was ductile. The yield of product amounted to 92.5% and the current yield to 82%.

Example 4 A melt consisting of 460 grams of KgTaFq and 1075 grams of an equimolecular mixture of sodium chloride and potassium chloride was electrolysed in an electrolytic cell of which the anode consisted of a graphite crucible of 80 millimetres internal diameter having a depth of 210 millimetres, and the cathode of a nickel rod of 15 millimetres diameter, the non-immersed portion of the rod being protected by a graphite tube, and the whole being shut off from the atmosphere by means of a quartz crucible. The bath temperature was 830 C., the current strength was 66 amperes, which corresponded to an anodic current density of 16 amp./dm. At the outset the voltage between the leads to the electrodes was 3.3 volts, and increased to 3.9 volts after 165 minutes. After 165.5 minutes the voltage suddenly rose very rapidly (anode effect), and then the electrolysis Was discontinued. The metal powder deposited at the cathode was recovered in the manner described in Example 1, and consisted of 201 grams of tantalum amounting to 94.8% of the theoretical yield. The current yield calculated on the tantalum obtained was 82%. The tantalum was coarsegrained and very ductile.

Example 5 A melt consisting of 460 grams of KzTaFq and 1075 grams of equimolecular mixture of sodium chloride and potassium chloride was electrolysed at a temperature of 750 C. in a cell of the kind used in Example 4. The electrolysis was carried out first for 26 minutes at a current strength of 250 amperes. After 26 minutes the anode eifect occurred, and then form the 26th minute to the 38th minute the current was lowered to 150 am-' peres, then from the 28th minute to the 52nd minute to amperes, and from the 52nd minute to the 88th minute to 50 amperes. After 88 minutes the voltage increased to 4.2 volts, that is to say, the voltage at which sodium is deposited under the conditions used, and this indicated the termination of the electrolysis.

By thus adjusting the current strength in accordance with the critical current density the duration of the electrolysis of the same quantity of potassium fiuow tantalate is shortened to 88 minutes as compared with 2165 minutes in Example 4, and therefore the output of tantalum in the cell per unit time is increased by 88%.

Example 6 Niobium was produced in the same apparatus used in Example 4, except that the anode and cathode regions were separated from one another by means of a graphite tube of 60 millimetres external diameter and having a wall thickness of 3 millimetres immersed in the melt and supported from above, therlower extremity of the tube being spaced about 20 millimetres from the bottom of the crucible. This graphite tube served as a diaphragm. A melt consisting of 1120 grams of an equimolecular mixture of sodium chloride and potassium carbonate, and 280 grams of K2NbF7 was introduced into the bath. The temperature of the melt was 750 C. and the current strength 60 amperes. At the outset the Voltage of the bath was 4.6 and increased after minutes to 5.2

volts.

After removing the deposit from the cathode, there was obtained a yield of metal amounting to 53.6 grams in the form of dendritic agglomerates. This corresponds to a product yield of 63% and a current yield of 82%.

Example 7 In a cell of substantially the same fiorm as that used in Examples 1-3, but having an internal diameter of 40 millimetres and a depth of 70 millimetres, and with the A melt consisting of 13 grams of K2T3F7, 24.1 grams of BaCl 13.2 grams of NaCl and 14.7 grams of KCl was introduced into an electrolytic cell of the kind used in Example 7. The temperature of the melt was 750 C. and the current strength 9 amperes. The bath voltage was initially 2.7 volts and it increased after 30 minutes to 3.7 volts. There was obtained a metal yield of 65% at a current yield of 65%,

What is claimed is:

1. A process for the production of a member selected from the group consisting of metallic niobium and metallic tantalum by the electrolytic method, wherein a double fluoride of the metal being produced is electrolyzed in a melt of a member selected from the group consisting of an oxygen-free alkali metal halide, an oxygenfree alkaline earth metal halide and a mixture of said oxygen-free halides at a temperature within the range of 650950 C. at an anodic current density not exceeding 500 amperes per square decimeter, and the anodic current density is reduced during the process of 7 electrolysis as the concentration of alkali metal fluoride in the melt increases, the adjusted current density being a function of the inverse of the fluoride concentration.

2. A process as claimed in claim 1, wherein the electrolysis is carried out at a temperature within the range of 750-900 C.

3. A process as claimed in claim 1, wherein the electrolysis is carried out with the exclusion of air.

4. A process as claimed in claim 1, wherein the anode zone and cathode zone are separated from one another by means of a diaphragm.

5. A process for the production of a member selected from the group consisting of metallic niobium and metallic tantalum by the electrolytic method, wherein a double fluoride of the metal being produced is electrolyzed in a melt of a member selected from the group consisting of an oxygen-free alkali metal halide, an oxygen-free alkaline earth metal halide and a mixture of said oxygen-free halides at a temperature within the range of 650-950 C. at an anodic current density not exceeding 500 amperes per square decinieter, and the anodic current density is continuously reduced during the process of electrolysis as the concentration of alkali metal fluoride in the melt increases, the adjusted current density being kept below the critical value given in amperes per square decirneter by T+5.6(T750) T750 where T is the temperature in degrees centigrade, e is the Euler number and A is the percentage in weight of alkali metal fluoride in the melt,

6. A process for the production of a member selected from the group consisting of metallic niobium and metallic tantalum by the electrolytic method, wherein a double fluoride of the metal being produced is electrolyzed in a melt of a member selected firom the group consisting of an oxygen-free alkali metal halide, an oxygen-ree alkaline earth metal halide and a mixture of said oxygen-free halides at a temperature within the range of 650950 C, at an anodic current density not exceeding 500 amperes per square dosimeter, and the anodic current density is continuously reduced during the process of electrolysis as the concentration of alkali metal fluoride in the melt increases, the adjusted current density being kept below the critical value given in amperes per square decimeter by where T is the temperature in degrees centigrade, and A is the percentage in weight of alkali metal fluoride in the melt and is at most 7.0%.

7. A process for the production of a member selected from the group consisting of metallic niobium and metallic tantalum by the electrolytic method, wherein a double fluoride of the metal being produced is electrolyzed in a melt of a member selected from the group consisting of an oxygen-free alkali metal halide, an oxygen-free alkaline earth metal halide and a mixture of said oxygen-free halides at a temperature within the range of 650-950 C, at an anodic current density not exceeding 500 amperes per square decimeter, and the anodic current density is continuously reduced during the process of electrolysis as the concentration of alkali metal fluoride in the melt increases, the adjusted ourrent density being kept below the critical value given in amperes per square decimeter by where T i the temperature in degrees centigrade, and A is the percentage in weight of alkali metal fluoride in the melt and is above 6.8% and below 18.1%.

8. A process for the production of a member selected from the group consisting of metallic niobium and metallic tantalum by the electrolytic method, wherein a double fluoride of the metal being produced is electrolyzed in an oxide-free melt of sodium chloride and of potassium chloride at a temperature within the range of 650-950 C. at an anodic currentdensity not exceeding 500 amperes per square decimeter, and the anodic current density is continuously reduced during the process of electrolysis as the concentration of alkali metal fluoride in the melt increases, the adjusted current density being a function of the inverse of the fluoride concentration.

9. A process for the production of a member selected from the group consisting of metallic niobium and metallic tantalum by the electrolytic method, wherein a double fluoride of the metal being produced is electrolyzed in a melt of a member selected from the group consisting of an oxygen-free alkali metal halide, an oxygen-free alkaline earth metal halide and a mixture of said oxygen-free halides at a temperature within the range of 650-950 C. at an anodic current density not exceeding 500 ampere per square decirneter, and the anodic current density is continuously reduced during the process of electrolysis as the concentration of alkali metal fluoride in the melt increases, the adjusted current density being a function of the inverse of the fluoride concentration.

10. A process for the production of a member selected from the group consisting of metallic niobium and metallic tantalum by the electrolytic method, wherein a double fluoride of the metal being produced is electrolyzed in an oxide-free melt containing at least one member selected from the group consisting of an alkali metal chloride, alkaline earth chloride and a mixture thereof at a temperature within the range of 650-950" C. at an anodic current density not exceeding 509 amperes per square decimeter, and the anodic current den sity is continuously reduced during the process of electrolysis as the concentration of alkali metal fluoride in the melt increases, the adjusted current density being a function of the inverse of the fluoride concentration.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENC3355 Metal Industry, June 29, 1945, pp. 406407. in Sci. Libr.) 

1. A PROCESS FOR THE PRODUCTION OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF METALLIC NIOBIUM AND METALLIC TANTALUM BY THE ELECTROLYTIC METHOD, WHEREIN A DOUBLE FLORIDE OF THE METAL BEING PRODUCED IN ELECTROLYZED IN A MELT OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF AN OXYGEN-FREE ALKALI METAL HALIDE, AN OXYGEN-FREE ALKALINE EARTH METAL HALIDE AND A MIXTURE OF SAID OXYGEN-FREE HALIDES AT A TEMPERATURE WITHIN THE RANGE OF 650-950* C. AT AN ANODIC CURRENT DENSITY NOT EXCEEDING 500 AMPERES PER SQUARE DECIMETER, AND THE ANODIC CURRENT DENSITY IS REDUCED DURING THE PROCESS OF ELECTROLYSIS AS THE CONCENTRATION OF ALKALI METAL FLUORIDE IN THE MELT INCREASES, THE ADJUSTED CURRENT DENSITY BEING A FUNCTION OF THE INVERSE OF THE FLUORIDE CONCENTRATION. 